The invention relates to the treatment of mechanical pulp waste effluent. More specifically, the invention relates to the application of freeze crystallization to purify mechanical plant effluent waste streams and to reclaim high purity water.
Mechanical pulp plant mills discharge waste streams which contain unacceptable quantities of contaminants such as inorganic salts, wood waste, organic materials, and volatile gases. The contaminants are commonly removed by concentrating the waste stream and separating the reclaimed effluent. Evaporation is a common concentration technique.
In evaporation, heat is applied to the waste stream to distill the reclaimed water. The heat, however, also distills contaminating organics and gases with the water. These contaminants are unacceptable, and secondary treatment of the distillate is necessary to recover high purity water. Thus, process water can be reclaimed only after these contaminants are removed from the distillate. After secondary treatment, reclaimed water can then be recycled to the pulping process, used in other processes, or discharged back to the environment.
It has been found, however, that mechanical pulp waste streams may be sufficiently concentrated by freezing the effluent. After freezing and separation, the resulting reclaimed water effluent is substantially higher in quality than the distillate from an evaporating process. Thus, the frozen reclaimed water effluent requires only minimal or no secondary treatment.
Accordingly, one advantage of the invention is that distillation of contaminants is avoided, thereby allowing high purity water to be recovered from mechanical pulp plant effluents. Further, the chemical oxygen demand (COD) in the reclaimed water may be brought down to zero more economically than known before. Also, low temperature treatment of the waste effluent reduces or eliminates corrosion of vessels and pumps experienced in the high temperature evaporator process.